1. Field of the Invention
The present invention relates to a fixing device for fixing a toner image on a to-be-fixed medium, on which the toner image is to be fixed, and to a control method for the fixing device, in an image forming apparatus such as an electrostatic copying machine or a laser printer.
2. Description of the Related Art
Jpn. Pat. Appln. KOKAI Publication No. 8-76620, for instance, proposes a fixing device using induction heating, instead of using a halogen lamp or a heat-resistant film. In the fixing device, an electrically conductive film is heated by magnetic field generating means, and toner is fixed on a paper sheet put in close contact with the conductive film. A heat-producing belt (electrically conductive film) is clamped between a member constituting the magnetic field generating means, on the one hand, and a heating roller, on the other, thus creating a nip.
Jpn. Pat. Appln. KOKAI Publication No. 9-258586 proposes a fixing device of the type that uses a heat-producing element formed by winding a coil around a core extending along the rotational axis of a fixing roller, and causing an eddy current to flow in the fixing roller, thus heating the fixing roller.
There is known a conventional fixing device of the above-described induction-heating type, which may operate in a mode for enhancing a fixing ratio. This operation mode, however, may enhance the fixing ratio by lowering a copy speed or raising a fixing roller temperature. As a result, the productivity may deteriorate, or the life of the fixing device decreases due to temperature degradation of consumable parts. On the other hand, in recent years, from the standpoint of energy-saving, it is desired to decrease the amount of energy that is uselessly consumed when there is no need to enhance the fixing ratio.